86 CONTRACTILE ELEMENTS. 



convulsions, is what has been called a physiological tetanus 

 (Ed. Weber). Thirty excitations a second are usually re- 

 quired to produce this. This result leads us to believe that 

 the contracted muscle, as we generally find it in the living 

 animal, remains for a certain time in the second form, only 

 by means of a succession of continuous shocks ; in fact, if a 

 muscle in this state be ausculted, a sound is heard, the mus- 

 cular tone, the height of which corresponds to about thirty 

 vibrations a second, which is exactly the number of excita- 

 tions, and, consequently, of muscular shocks, needed to pre- 

 serve the second form, or experimental physiological tetanus 

 (Wollaston, Helmoltz). 



If, by means of thirty excitations a second, the shocks be 

 rendered simultaneous, producing permanent contraction (or 

 physiological tetanus), and the rapidity of the excitations be 

 then increased, the force of the contraction increases also / 

 and, since the tone, or muscular sound, becomes sharper and 

 higher, it is proved that the contraction is composed of a 

 greater number of blended jerks or shocks. This may be 

 easily verified by listening one's self to the sound of the 

 masseter when more or less strongly contracted. This sound, 

 heard in the perfect stillness of the night, sometimes rises as 

 high as a fifth (Marey). 



If the muscle be fatigued, the shocks are more easily 

 blended together, but the force of the contraction is dimin- 

 ished (Marey). 



There are certain striated muscles in which the shock 

 takes place very slowly; in other words, their curve of con- 

 traction is very long. Such is the case with the muscles of 

 the tortoise and the muscular fibres of the heart (Marey). 

 The latter form a sort of transition between the striated 

 and the smooth muscles, the shock of which is very long, 

 and is represented in a diagram by the line of physiological 

 tetanus. 



If a weight be attached to the extremity of a muscle at 

 the instant when a shock takes place, or during the physio- 

 logical tetanus, the weight will be raised, unless it be too 

 great (see pp. 75 and 76). This constitutes the labor of the 

 muscle, and it is in this way that its force is measured. 



The height to which a muscle can lift any weight depends 

 on the length of its fibres ; but what is meant by its force of 

 contraction (absolute muscular force) is measured by the 

 weight requisite for the neutralization of the movement, and 

 depends only on the extent of the transverse section of the 



